1. Field of the Invention
This invention relates to an optical fiber temperature sensing device that is adapted to measure ambient temperature by detecting Raman scattered light scattered in an optical fiber and, in particular, to an optical fiber temperature sensing device that uses information obtained from the detected Raman scattered light to control output of a light source thereof and detect a malfunction of the device itself.
2. Description of the Related Art
As shown in FIG. 3, a conventional optical fiber temperature sensing device 31 is composed such that a signal processing control circuit 33 of a sensor body 32 generates a pulse signal, an LD (semiconductor laser) module 34 converts the pulse signal into a pulse light signal (with an input wavelength λ0), the pulse light signal is inputted through an inside optical fiber 35a to an optical filter 36, and the filtered pulse light signal is outputted through a reference temperature optical fiber 37 to a measuring optical fiber 35b. Hereupon, backscattered light is generated from each point of the measuring optical fiber 35b. 
The backscattered light includes Rayleigh scattered light with a transmission wavelength of λ0, and Stokes light (herein called “St light”) with a wavelength of λSt and anti-Stokes light (herein called “As light”) with a wavelength of λAs, where the St light and As light compose the Raman scattered light (See FIG. 4).
The St light and the As light are separated from the backscattered light by the optical filter 36, they are each received by optical receivers 38 to be converted into electrical signals, and the signals are then inputted to the signal processing control circuit 33. The signal processing control circuit 33 operates to process the electrical signals to calculate an intensity ratio of the St light and the As light, and to calculate temperature at a measurement site from the intensity ratio. The temperature data calculated is transmitted from the signal processing control circuit 33 to a personal computer 39 to allow the data to be displayed thereon.
The device 31 includes the reference temperature optical fiber 37 housed in an optical fiber housing box 40, and a temperature sensor 41 attached to the reference temperature optical fiber 37. The temperature sensor 41 is connected to the signal processing control circuit 33.
Since the intensity ratio of the St light and the As light is dependent on the ambient temperature, a temperature distribution around the measuring optical fiber 35b can be determined by the measuring the intensity ratio by the optical fiber temperature sensing device 31.
However, an LD used in the LD module 34 changes in output depending on ambient temperature. Therefore, it is necessary to keep the output of the LD constant during the measurement in order to accurately measure the temperature.
It may be assumed that the following methods for keeping the output of the LD constant are applicable to the optical fiber temperature sensing device 31.
(i) To provide the LD module 34 with a monitoring PD to detect a light emitted in the opposite direction to the emission direction of the LD, so that the signal processing control circuit 33 can control the output of the LD to be constant based on the output of the monitoring PD, and
(ii) To provide the LD module 34 with a peltiert device having a temperature-adjusting function to control the temperature of the LD to keep the output of the LD constant.
However, if the above methods (i), (ii) are applied to the conventional optical fiber temperature sensing device 31, in case of the method (i), the monitoring LD has to be added to the LD module 34, and in case of the method (ii), the peltiert device has to be added to the LD module 34. Thus, in either case, a problem arises that the whole cost of the optical fiber temperature sensing device 31 increases due to the addition of the device.
Another problem arises that the whole construction of the optical fiber temperature sensing device 31 is complicated when the LD module 34 is provided with the monitoring PD or the peltiert device.
A further problem arises that the structure of the signal processing control circuit 33 is complicated since the signal processing control circuit 33 needs to process signals from the monitor PD or the peltiert device as well as signals for the St light and the As light.
In case of detecting a malfunction of the LD, the monitoring PD will be needed so that the same problems as above can be caused.
Related art to the invention is, e.g., JP-B-2577199.